The present invention relates to a heat sealing jaw assembly and method of operation having a pair of metal jaw members each having a heat sealing member of soft material and capable of conducting heat to a fusing temperature of juxtaposed heat sealable sheet material disposed between the jaw members. More specifically, but not exclusively, such heat sealing jaw assembly is for use in sealing juxtaposed thermoplastic film material in a form, fill and seal machine.
Various types of heat sealing jaw assemblies are known in the prior art for sealing juxtaposed film materials, such as thermoplastic film to form sealed packages. These packages or pouches, may be preformed and then later filled with a product and then sealed or else the pouches can be formed simultaneously as they are being filled such as in a form, fill and seal machine. U.S. Pat. No. 5,326,416 is referenced for describing such a heat sealing assembly in a vertical form, fill and seal machine for packaging flowable materials. Such assemblies comprise two jaws, resilient clamping elements and a heat sealing element. At least one of the jaws is capable of transverse motion and adapted to collapse a tubular film made from thermoplastic film material passing between the jaws. The clamping assembly is adapted to grip thermoplastic film which passes therebetween and urge the grip film towards the jaws when the jaw closes and release the film when the jaw opens. One of the jaws is provided with a heat sealing element secured to the clamping face and the other jaw is provided with a heat resistant resilient pad, such as silicone rubber.
When the film tube is collapsed there are two juxtaposed film sheets. To fuse the juxtaposed film sheet, heat must pass from an outer face of one of the film sheets and through the other sheet and to its back face to achieve a complete seal. As the seal is formed from one side of the first film sheet to the outer side of the second film sheet, the temperature of the sealing element falls drastically. Therefore, it is necessary that the sealing element be hot enough to achieve the complete seal across both film sheets. Therefore, the first film sheet is subjected to a higher sealing or melting temperature than the second sheet and this often causes imperfect seals and leakage of the contents results. It can also result in machine stoppage and this becomes very costly. The speed of the sealing operation of the machine is also limited by the speed or fusing time of the sealing jaws.
Another disadvantage of prior art sealing jaws is that the sealing element is constructed of metal and a layer of woven glass fiber cloth is usually disposed thereover. This makes for a very rigid sealing face structure and if there is a crease in the juxtaposed film sheets, the film may be damaged by the rigid sealing head which does not absorb the imperfections in the film caused by the creases and the film could be punctured. Furthermore, the prior art sealing heads usually have a pointed end or are provided with a hot wire whereby the seal and the slitting is formed simultaneously. With a sealing head having an integrally formed pointed slitting head, the temperature at the slitting point is substantially the same as the temperature of the sealing side walls of the head forming the seal. Accordingly, the temperature of the slitting element is not controlled independently of the sealing head and again this may result in an imperfect seal and leakage.
It is a feature of the present invention to provide a heat sealing jaw assembly and method of operation which overcomes the above-mentioned disadvantages of the prior art and which can achieve better seal performance at much greater sealing speeds.
Another feature of the present invention is to provide a heat sealing jaw assembly and method of operation wherein the temperature of the sealing heads and the cutting or slitting element are independently controlled.
Another feature of the present invention is to provide a heat sealing jaw assembly and method of operation and wherein the sealing jaw members are each provided with a ribbon member of soft material having rubber-like properties and which is capable of conducting heat from the sealing jaw to a fusing temperature of at least two juxtaposed heat sealable sheet materials and wherein a seal is formed from opposed sides of the juxtaposed sheets.
According to the above features, from a broad aspect, the present invention provides a heat sealing jaw assembly for effecting a heat seal of at least two juxtaposed heat sealable sheet materials. The heat sealing jaw assembly comprises a first and a second metal jaw member each having a heating element for heating the jaw members. Each jaw member has an end face disposed in facial alignment with one another. A heat conductive sealing member is secured to each of the end faces and disposed in planar alignment with one another. The heat sealing member is of a soft material having rubber-like properties and capable of conducting heat from the sealing jaw members to a fusing temperature to melt the heat sealable sheet materials. The first and second jaw members are reciprocable towards and away from one another to press the aligned heated sealing members against opposed sides of at least two juxtaposed heat sealable sheet materials disposed between the jaw members to effect a heat seal from opposed sides of the juxtaposed sheets.
According to a further broad aspect of the present invention, there is provided a cutting metal slitting wire disposed forwardly and substantially centrally along one of the heat sealing members and heated independently to effect a cut in the heat seal.
According to a further broad aspect of the present invention the temperature of the heat sealing metal jaw members are controlled independently from one another.
According to a further broad aspect of the present invention there is provided a method of forming a heat seal between juxtaposed heat sealable sheet material, said method comprising the steps of: (i) providing a pair of metal jaw members having heating means for heating same to a desirable temperature, each said jaw member having an end face with said end faces disposed in facial alignment, means to reciprocate said jaws towards and away from one another, (ii) providing a heat conductive sealing member of soft rubber-like material along at least a common section of each said end faces in contact therewith to conduct heat from said jaw members to a fusing temperature to melt said heat sealable sheet materials, and (iii) displacing said jaw members towards one another to press said heat conductive sealing members against said juxtaposed heat sealable sheet material from opposed sides of said sheet material and with said heat conductive sealing members in planar alignment with one another to fuse said materials and seal them together along a seal line and from opposed sides of the juxtaposed sheets.